The present invention relates to a method of manufacturing an electronic component.
As an assembly structure of an electronic component, it is known to bond a wire for connecting a substrate and a semiconductor element on an electrode on the surface of the substrate and to form a bump as a protruding electrode on other electrode. As such electronic component, a BGA (ball grid array) package is known. As the wire, a gold wire is used widely, and as the bump, a solder is used widely.
FIG. 17 is a sectional view of a conventional substrate. In the diagram, reference numeral 1 denotes a substrate such as glass epoxy substrate, and copper pads 2, 3 of circuit patterns are formed on its upper and lower surfaces. Nickel layers 4, 5 are formed on the copper pads 2, 3, and gold layers 6, 7 are formed on the nickel layers 4, 5. The upper side copper pad 2 and lower side copper pad 3 are connected by an internal wiring 8. On the gold layer 6 of one copper pad 2, the end of a gold wire 9 for connecting a semiconductor element (not shown) mounted on the substrate 1 is bonded, and a solder bump 10 is formed on the gold layer 7 of other copper pad 3, thereby assembling an electronic component.
The nickel layers 4, 5 and gold layers 6, 7 are generally formed by plating method. The gold layers 6, 7 are formed for enhancing bonding performance of the gold wire 9. Conventionally, the thickness of the gold layers 6, 7 was about 0.2 to 1 micron, and they were considerably thick. The nickel layers 4, 5 are formed as barrier metal for preventing the material copper of the copper pads 2, 3 from diffusing into the gold layers 6, 7.
In such conventional method, when plating with gold, the component nickel of nickel layers 4, 5 is melted into the plating solution, and the nickel mixes into gold plating layers. The nickel positioned on the surface of the gold layers 6, 7 forms a compound such as oxide and hydroxide by heating in the process of fixing the semiconductor element. This compound blocks bonding of gold wire 9. As the gold layer becomes thicker, an amount of nickel positioned on the surface is reduced, and in order not to form this oxide as far as possible, the thickness of the gold layers 6, 7 must be greater than a certain thickness.
However, if the gold layers 6, 7 are too thick, bonding force of solder bump 10 drops. This is because, when forming the solder bump 10, the gold in the gold layer 7 is melted into the solder bump 10 and is combined with tin in the solder to form a brittle compound. Thus, for the purpose of bonding of gold wire 9, one gold layer 6 is preferred to be thick, while the other gold layer 7 is preferred to be thin for forming and soldering the solder bump 10. It is hence difficult to solve these contradictory problems.
It may be also considered to apply gold plating only on the electrode for bonding the gold wire 9, but it requires a process of covering the non-plating electrode with a mask, and a process of removing this mask after plating, and the manufacturing cost is much increased.
It is hence an object of the invention to present a method of manufacturing an electronic component capable of bonding a gold wire favorably on an electrode on a substrate and soldering favorably.